Metal-Organic Frameworks (MOFs) are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, the pores are stable to elimination of the guest molecules (often solvents) and can be used for the storage of gases such as hydrogen and carbon dioxide. Other possible applications of MOFs are gas purification, gas separation, catalysis and sensors.
With the emergence and development of new materials, such as nanotubes or MOFs, it is important to determine the gas adsorption characteristics of these materials, especially under high pressure and/or in a gas mixture. The characterization of gas adsorption in porous materials is performed predominantly by commercial gravimetric equipment. However, their capital and operating costs are generally high. Furthermore, they require relatively large amounts of sample (typically >100 mg) to obtain accurate data, and also cannot measure gas adsorption in thin films or coatings. It is therefore desirable to broaden the range of techniques that can be used to reliable measure the adsorption properties of MOF materials over a substantial range of pressure and temperature by non-gravimetric methods, ideally with only a small sample size requirement (<1 mg), and with the sample being potentially in powder, coating, or thin film form. The ability to measure the gas adsorption and desorption characteristics of thin films and coatings, or very small amounts of sample, under high pressure and temperature, and/or in an environment containing more than one gas, is desirable because that is usually the real conditions under which the thin films/membranes/coatings are to be used.
Quartz is one member of a family of crystals that experience the piezoelectric effect—the charge that accumulates in certain solid materials (notably crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical strain. The piezoelectric effect has found applications in high power sources, sensors, actuators, frequency standards, motors, etc., and the relationship between applied voltage and mechanical deformation is well known, and allows probing of acoustic resonance by electrical means.
A quartz crystal microbalance commonly referred to as a “QCM”, measures a mass per unit area by measuring the change in frequency of such a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to oxide growth/decay or film deposition at the surface of the acoustic resonator. The QCM can be used under vacuum, in gas phase, and more recently in liquid environments. Frequency measurements are easily made to high precision. In addition to measuring the frequency, the dissipation is often measured to help analysis. The dissipation is a parameter quantifying the damping in the system, and is related to the sample's viscoelastic properties. Other advantages of QCM over conventional gravimetric devices are that it is compact and simple for system set-up, stable for in situ measurements (no buoyancy effect), and exquisitely sensitive.
Although highly sensitive, the QCM has not been typically used in high pressure environments because pressure changes affect the frequency of oscillation, thus complicating interpretation of the measurements. Also, quartz can be brittle and may crack under shock or sudden changes in pressure. WO2009108825 discloses a high pressure QCM, but the device therein has only a single QCM in the high pressure chamber, and thus suffers from the problem of accurate calibration for quantitatively correct measurements of adsorption. The response to temperature and pressure has to be first evaluated without the presence of analyte, and then repeated under identical conditions with analyte present. However, the reproducibility in setting the temperature and pressure can complicate interpretation of the data. Therefore, the apparatus described in the patent WO2009108825 can only provide qualitatively correct data.
U.S. Pat. No. 7,036,375 uses a plurality of piezoelectric transducers, as does the art referenced therein, wherein each of the transducers can have two oscillating domains, one used as a reference and the other being used as a target oscillating domain for measurement of a sample. See also U.S. Pat. No. 6,544,478. However, in these patents the devices are multi-channel QCM sensors designed to allow measurement of more than one sample at a time and do not relate to high pressure QCMs. Furthermore, these QCM sensor arrays operate close to room temperature in an aqueous environment for the detection of adsorbed biomolecules. The presently described device has been tested to operate at a range of temperatures from room temperature to about 185° C. under low vacuum, and under gas environments from room temperature to 120° C. for pressures tested up to 8.25 atm so far.
Therefore, there is a need for a better apparatus capable of measuring the adsorption and desorption characteristics of a material under high pressure and/or in an environment of one or more gases.